The thermostat on the 2001 Nisasan Altima sits inside the intake housing, on the lower half of the engine. The main function of the thermostat is to regulate the operating temperature of the coolant. When the coolant gets too hot, a spring located inside the thermostat compresses and lets more fluid flow through the radiator. As air hits the radiator fins, it cools the fluid passing through the radiator; in turn, the fluid cools off the engine. When the thermostat breaks, this process stops happening and you risk overheating the engine.
Raise the front end of the vehicle with either a set of ramps or jack stands. Secure the vehicle with the parking brake.
Place a drain pan under the radiator and open the petcock to drain all of the fluid. Use a pair of pliers to open the petcock if you cannot do so with your fingers. Close the petcock once the fluid completely drains out.
Trace the lower radiator hose to the engine block--the hose connects to the thermostat intake housing, which is bolted to the engine block. Remove the band clamp that holds the hose to the housing, using a screwdriver. With the clamp removed, pull the hose off the intake housing.
Remove the bolts that hold the intake housing onto the engine block, using a socket and ratchet. Hold the thermostat housing with one hand while you remove the bolts. The housing tends to fall off the block, taking the thermostat and gasket with it--if the housing hits the ground, it can crack.
Pull the thermostat, gasket and O-Ring out of the housing. Place the new O-Ring into the groove on the bottom of the housing. Place the thermostat into the housing with the giggle valve (end with the pin on it) sticking into the thermostat housing. Place the gasket on the bottom of the housing and hold the housing onto the engine block; secure it with the bolts.
Slide the radiator hose onto the housing and secure it with the band clamp.
Drain a 50/50 mixture of radiator fluid and distilled water into the filler spout of the radiator.
